The field of the disclosure relates generally to electric motors, and more specifically, to protecting electric motor control circuits from transient and/or over-voltage conditions.
Electric motors are used in countless varieties and applications worldwide. For example, electric motors are included within heating, ventilation, and air conditioning (HVAC) applications. Such electric motors include motor controllers for controlling the electric motors. Theses motor controllers include drive circuits for sending operational signals to drive the electric motors. These motor controllers may be subject to transient voltages and/or over-voltage conditions that can damage motor controller components and/or the electric motor.
Transient voltages may be caused by events external to the HVAC application, for example, power surges. Power surges may be caused by numerous different external forces, most typically, from lightning strikes. Transient voltages generally produce a voltage surge of up to 6000 volts. Typical protection devices for transient voltage protection include clamping circuits positioned in the filter circuit between the power supply and the motor controller. More specifically, some known protection devices connect metal oxide varistors (MOV) in a “Y” connection across the three-phase power lines. However, the clamping voltage of MOVs are usually twice the maximum allowable direct current (DC) voltage, therefore the voltage rating of the rectifier in the motor controller must be at least twice the working DC voltage. This results in larger physical volume and higher costs. For example, in a 56 frame electric motor where VRMS=460V, twice line voltage leads to VDC=1400V. Thus, the clamping voltage of the MOV must be around 2800V. No rectifier currently exists in the market with such a voltage rating that can be fit in a 56 frame motor enclosure.
Over-voltage conditions typically occur when there are faults in capacitor banks used for power factor correction in the power distribution system. These faults may introduce DC line over-voltages up to twice the normal value of DC line voltages. These over-voltage conditions may burn fuses in electric motors or even cause damage to them. When fuses are burned, technicians must replace them and reset the machine, which is costly.